Complex growth microstructure in Bushveld aureole metapelite:
growing porphyroblasts pseudomorph the microstructure of the matrix
they replace. allowing the detailed sequence of growth to be
determined.

Metamorphic Studies Group Research in Progress 2010
(keynote);
Interplay between Thermodynamics, Kinetics and Deformation in Metamorphism,
GeoCanada 2010 (invited talk).

My daughter Helen drew this cartoon to illustrate the
principle of the title. Equilibrium, on the left, believes metamorphic
products should always follow ordered patterns governed by the logo on his
t-shirt. He looks disgruntled because Process, on the right, is busy
explaining why she won't do this metamorphism thing exactly the way he plans
it. She clearly has her own style, and while her successful products may
sometimes take unexpected forms, you can bet that she has found efficient
ways of making them, because she wouldn't tackle any job that involved too
much effort to get it started.

Equilibrium and Process seem like an ill-matched
couple, but they need each other, and we have to understand the point of
view of each of them in order to grasp the principles of metamorphism.

What follows is an extended abstract of the GeoCanada
lecture (with some updated figures).

Summary

Ongoing developments in thermochemical data sets and calculation software
have led to the routine use of calculated equilibrium phase diagrams, and in
particular P-T pseudosections, as a tool for investigating the P-T evolution
of metamorphic rocks. However, the real processes that
transform metamorphic rocks take place away from equilibrium, and are
governed by the kinetic controls on nucleation, interface reaction, and mass
transfer.

This contribution explores how the microstructural and
micro-compositional record in rocks may, or may not, be reconcilable with
calculated phase diagrams in the relevant systems. Examples are drawn from
metapelites in the High Himalaya, from the aureole of the Bushveld Complex,
and from Alpine metabasic eclogites. In practice we find that although the
predicted equilibrium assemblage is commonly approached, the reaction
pathways can be strongly affected by nucleation barriers, compositional
fractionation effects, and the slow decomposition of reactant phases.

Introduction

The advent of self-consistent data sets for systems closely relevant to
natural rocks and the steady improvement of software tools for calculating
phase diagrams (e.g. Powell et al., 1998) are transforming our ability to
visualize the equilibrium state of metamorphic systems. P-T pseudosections,
for example, are routinely used to constrain P-T conditions and infer
P-T-time paths.

The real processes that transform metamorphic rocks, however, take place
away from equilibrium, and are governed by kinetic constraints.
Non-equilibrium effects are potentially at play in all three fundamental
processes of prograde metamorphic change: barriers to nucleation of new
phases, sluggish rates of interface reaction, and limits to the
effectiveness of mass transfer. Moreover, the pseudosection approach
requires that the bulk composition is defined, and yet the effective
composition of the reacting system is governed by these same kinetic
factors. In particular, refractory porphyroblasts fractionate material from
the reacting system, and may also be slow to release material when they
become reactants. Illustrations of these effects are
taken from a range of case studies in which the microstructural and
micro-chemical record is compared with calculated phase relationships.

Case study 1: Everest metapelite

Metapelitic rocks in the footwall of the South Tibetan Detachment System
at Mt Everest contain a detailed record of their
metamorphic evolution. Microstructural analysis of a lower sillimanite zone
schist reveals that garnet growth entirely predates the main S2 matrix
fabric, which formed under staurolite-zone conditions. The record of
compositional zoning in garnet also reveals that it grew
only under garnet zone conditions on the trajectory shown by the hollow
arrow on Figure 1, the remainder of the path being deduced from other
data. In contrast to the equilibrium prediction, garnet
failed to contribute to staurolite growth, and ultimately experienced
corrosion under sillimanite-zone conditions without re-equilibration of the
garnet rims, despite the high peak temperature of around 640°C
(Figure 1). It follows that any attempt to calculate
"peak" P and T by conventional thermobarometry using the full assemblage
Grt-St-Sil-Bt-Ms-Pl-Qtz is doomed to failure.

Figure 1: P-T pseudosection in the system NCKFMASH for a
sillimanite-zone Everest Series metapelite. The hollow arrow shows the
growth interval of garnet calculated from compositional zoning profiles.

Case study 2: Aureole of the Bushveld Complex

Shales of the Timeball Hill Formation (Pretoria Group)
experienced a static but long drawn-out metamorphism with a peak at 550°C, 3
- 3.5 kbar in the thermal aureole beneath the 8 km thick mafic Rustenburg
Layered Series. Mineral growth microstructures, preserved in exquisite
detail, reveal that porphyroblastic staurolite, cordierite, biotite and
andalusite appeared in an overlapping sequence entirely at odds with the
equilibrium phase diagram (Figure 2), apparently as a consequence of delayed
nucleation of andalusite and the parallel operation of several metastable
reactions (Waters and Lovegrove, 2002). The full range of microstructures
can be attributed to nucleation barriers differing among the product phases,
evolution of the effective bulk composition (EBC) by fractionation of the
newly-grown porphyroblasts, and slow dissolution of reactant chloritoid
compared to matrix sheet silicates. Nevertheless, the final assemblage at
the metamorphic maximum approaches a possible equilibrium one.

Case study 3: Alpine metabasic eclogites

Eclogites from the Monviso unit, Italian Western Alps, shed further light
on fractionation processes and the evolution of a rock's effective bulk
composition. The dominant minerals are omphacitic clinopyroxene and garnet,
both of which are refractory and show compositional zoning related to growth
along a P-T trajectory. However, because the rocks were undergoing dynamic
recrystallisation, fractionation of mineral core compositions out of the EBC
is not fully effective. In garnet, networks of vein-like Mg-rich domains,
probably representing fluid pathways along healed fractures, penetrate and
transform early garnet cores, commonly leading to atoll structures. In
omphacite aggregates, migrating grain boundaries sweep across the interiors
of old grains, recycling core material back into the EBC. Estimating the EBC
at different stages of the P-T evolution is a challenging task!

Figure 3: Backscattered electron micrograph showing
growth-zoned omphacite in an aggregate that has undergone dynamic
recrystallisation. Grain at left advanced into the grain at right, its
medium-grey rim truncating the bright (Fe-rich) core and dark (Na-rich)
outer zone of the right-hand grain. Width of view approx 120 μm.

Conclusions

In practice, therefore, we find that although
calculated phase diagrams provide a valuable frame of reference for
interpreting the P-T evolution of metamorphic rocks, the actual
reaction pathways can be strongly affected by disequilibrium processes.
Moreover, the effective bulk composition of a rock undergoing metamorphism
is not always easily established, and may change significantly during the
P-T evolution. A detailed study of microstructural and
micro-compositional patterns may be needed to interpret the mineral
assemblage evolution, but a combination of this process-oriented
approach with equilibrium calculation can yield
unexpected insights into metamorphic processes.

Acknowledgements

I am grateful to the organizers of this special session for
their invitation to contribute, and especially to Dave Pattison for long
discussions on kinetic matters. Studies in the Everest Himalaya involve
collaboration with Mike Searle, Rick Law, John Cottle and Micah Jessup; Dan
Lovegrove contributed to the Bushveld aureole work; and Ryan Langdon and
Samuel Angiboust are involved with the ongoing study of the Monviso
eclogites.